A new assay for transformed cells is being developed using as the marker plasminogen activator. The basis of the assay is a new fluorogenic substrate which is an active site titrant of plasmin. This derivative of fluorescein is at least 20-fold more sensitive than presently available derivatives. The derivative has been shown to be an excellent active site titrant of trypsin, plasmin, thrombin, urokinase, factor Xa, factor IXa and has been used to characterize the activation of plasminogen by urokinase. To measure the plasminogen activator of transformed cells, 10,000 cells are incubated in microwells with purified plasminogen. The resultant plasmin is then titrated with the derivative. The assay is quick, sensitive, and quantitative. As few as 250 transformed cells can be assayed. The Km(app) and Vmax for the activation of plasminogen by transformed cells, in situ, can be determined. This assays is now being used to study the correlation between plasminogen activator activity and tumorigenicity.